Color and gray value images are both composed of picture elements (pixels), each pixel represented by multiple binary bits which define either a color or a gray level. In order to represent such an image on a bilevel (black/white) printer, the pixel data, if not already gray level, is converted into a gray level multi-bit value (i.e., 8 bits per pixel). The individual gray level pixels are then converted to binary level pixels through use of a scaling and dithering process. The dithering process provides a halftone-like texture to selected areas of the image so as to provide gray value variations therein.
The dither process compares individual pixel values against correspondingly located values in a threshold matrix, to control the conversion of the gray level values to appropriate patterns of bi-level data. When 8-bits are employed, 256 gray levels can be represented by the digital values. The threshold matrix comprises a plurality of row-arranged, gray level values which control the conversion of the gray level pixel values to binary pixel values. In essence, each entry in the threshold matrix is a threshold gray level value, which, if exceeded by the image gray level pixel value, causes that gray level image pixel to be converted to a black pixel. If, by contrast, the image gray level pixel value is less than or equal to corresponding threshold matrix gray level value, it is rendered as a "white" pixel.
During a dithering process, the threshold matrix is "tiled" across the image pixels to enable each gray level image pixel to be compared against a correspondingly, logically-positioned, gray level value in the threshold matrix.
Laser printing devices have now become available which are capable of providing "n" levels of gray (including white) at each pixel site, with n being greater than 2. Often, n is a value from 4 to 8. Hereafter, it is to be understood that when the term "gray" is used, it applies to both color and black/white images and, when applied to a color image, relates to the intensity of the color on a scale of 0 to N-1 where N is the greatest value obtainable from the number of bits used to represent the color. In the case of 8 bits, N=256.
Clearly, if a laser printer is able to produce all N levels of a color, then no need exists to halftone the image, as all gray levels can be fully represented in the rendering. However, there is a countervailing requirement to reduce printer costs. Thus, the amount of data used to represent a pixel is often limited to just a few bits per pixel, per color plane, to reduce the amount of required memory. Nevertheless, a requirement still exists for a compromise solution between continuous tone printing (having a high spatial frequency) and printing which employs a limited number of levels of gray (and exhibiting a lower spatial frequency).
Accordingly, it is an object of this invention to provide a dither/halftone procedure for a multi-gray level printer apparatus which enables the apparatus to produce near photographic quality images.
It is another object of this invention to provide an improved method for halftoning a multi-gray level image wherein marking engine artifacts, and rendering artifacts are minimized.
It is yet another object of this invention to provide an improved method and apparatus for halftoning a multi-gray level image wherein halftoning procedures are individually adjustable, for each of plural image color planes.